Q&A

Volcanologist Sheridan discusses ‘Popo’

By CHARLOTTE HSU

UB volcanologist Michael Sheridan began studying
Mexico’s Popocatepetl volcano in the mid-1990s. He was a
member of the team that created the first hazard map of the volcano
in 1995—a map that is still used today.

After a significant eruption in 2000, Sheridan worked with a
team of researchers to develop a new map showing which communities
around the volcano could be threatened by mud flows during a major
event.

In the following Q&A, Sheridan, professor emeritus of
geology, discusses why Popo, which been coughing up ash, gas and
molten rock over the past few days, is one of the world’s
most dangerous volcanoes and why it remains difficult for
researchers to predict major eruptions.

Why is Popocatepetl so dangerous?

MS: Hundreds of thousands of people live within the
“red zone” of this volcano, in an area where there is
significant risk during an eruption. This dense population makes it
a very dangerous volcano. There have been five huge eruptions at
Popo since the end of the last ice age, about 14,000 years ago. If
you visit some of the villages in the area, you can see that
volcanic mud flows covered whole fields in ancient times. During
one ancient eruption, blocks the size of a person’s fist
landed in Mexico City.

What’s the risk of a major eruption today?

MS: Today, what we see is that the volcano has a big
crater—really big; I mean, a mile wide and thousands of feet
deep. That crater has been slowly filling with lava. The longer it
builds up, the more dangerous it is because it becomes easier for
an explosion within the crater to throw material out of the crater
onto the steep volcano flanks where it can slide down the slopes
toward inhabited areas. Major events like this have happened in the
past, so they could happen again. What’s difficult to predict
is precisely when they will occur.

Why is it so hard to predict the nature of eruptions at Popo?

MS: It’s like a doctor diagnosing an illness. Once
you have seen several cases of the same illness, you can see the
pattern of symptoms in a patient leading up to the most severe
conditions. You can forecast how the patient will progress. For
Popo, we don’t have that kind of data. The last major
eruption happened before we had the sophisticated instruments we
have now, so we don’t have any record of the tremors, gas
emission and ash-ejection patterns that preceded major events. Some
volcanoes—including in Hawaii—have been erupting
continuously and have had an observatory on them for a century. In
these places, scientists are able to look at wiggles on seismograms
and make a prediction about what will happen. That’s not the
case for Popo.

Please talk a little bit about the maps you created.

MS: I participated in drafting two maps of Popo. The
first map was completed in 1995 by an international team during the
first months of the volcanic crisis. In 2000, we made a second map
using computers to simulate small- and intermediate-sized mud flows
at Popo because this appeared to be one of the important hazards
that would develop with time. Using this information, we created a
map showing where mud flows might travel during an event. Such maps
can’t be absolutely accurate because they represent a
conceptual computer model, but they can be a helpful tool in
demonstrating the danger that this volcano poses.

What are some ways that people can protect themselves from volcanic mud flows?

MS: They can identify places where they can be safe. In
the event of mud flows, people should find higher ground and stay
away from dry streambeds where mud could travel. That higher ground
may be in the cemetery, because bodies are often buried in very dry
places on a hill. It takes the mud some time to get down the slopes
of the volcano, so people may have as much as half an hour to get
to higher ground. If a mud flow is coming, you shouldn’t
spend a lot of time gathering valuables. You can get out of the
way, but you won’t be able to save much. Just save
yourself.